Writing to Stephen Powis, medical director at the Royal Free NHS Trust, Caldicott particularly criticised the justification for the data sharing, saying it was implied that it was for patient care - but she noted that's not what the Streams trial was actually about.
However, Caldicott said that this basis may not have been valid in the arrangement between the Royal Free Hospital.
The development raises fresh concerns about how the NHS handles patients' data after last week's cyberattack on hospitals and GP surgeries, which could have been prevented if staff had followed guidance issued a month earlier.
The NDG-which is responsible for advising and challenging healthcare systems to ensure patients" data is safeguarded and utilised correctly-believes that it was "inappropriate' for the hospital to provide DeepMind with this information without asking patients' permission first.
Caldicot's assessment will be passed on to the Information Commissioner's Office, which is investigating the agreement under data protection rules.
Under common law patients are "implied" to have consented to their information being shared if it was shared for the objective of "direct care".
Starting in July 2015, the Royal Free worked with DeepMind to develop an app that tells doctors when a patient is at risk of developing acute kidney injury. This condition is linked to 40,000 deaths a year in the United Kingdom, with the NHS estimating that a quarter of these are preventable.
Dame Fiona did not dispute the value of the app for patients, but in the letter to Prof Powis she explained that in her "considered opinion" the "purpose for the transfer of 1.6 million identifiable patient records to Google DeepMind was for the testing of the Streams application, and not for the provision of direct care to patients".
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"My considered opinion therefore remains that it would not have been within the reasonable expectation of patients that their records would have been shared for this goal", she says.
The UK's privacy watchdog the Information Commissioner's Office (ICO) is now investigating whether the transfer was legal, and expects to reach a decision soon.
The ICO told Sky News that its investigation "is close to conclusion".
Update May 16th, 05:00AM ET: Updated with additional comment from DeepMind and the Royal Free. The ICO has the power to levy fines of up to £500,000 on organizations that break data protection laws. It is now in use at the Royal Free, and is helping clinicians provide better, faster care to our patients.
Both the Royal Free Hospital and Google DeepMind were first informed of Dame Fiona's opinion in December.
Nicola Perrin, head of Understanding Patient Data, said such technologies offer potential to provide better patient care, "but there must be appropriate governance so that everyone can have confidence that patient data is being used responsibly". "Patients should know how their data is used, including for possible improvements to care using new digital tools". The ICO is looking into whether the Royal Free illegally transferred patient data to DeepMind. He also asserted that DeepMind is a British company, which still operates independently even after being acquired by Google.
With many going further believing that google trying to worm it way into the NHS will help it create something similar to the mark of the beast using patients data to deny treatment to patients GOOGLE believes does not deserve to be treated. However, not every patient was aware that their data was being given to Google.
Sky News has today obtained and published (embedded in the below tweet) a letter sent by the NDG to the Royal Free NHS Trust and to DeepMind co-founder Mustafa Suleyman.